1. Technical Field
The present invention relates to a field effect transistor, a display device, a sensor, and a method of fabricating a field effect transistor.
2. Related Art
Field effect transistors are widely used as unit elements of integrated circuits for semiconductor memories, high-frequency signal amplifying elements, elements for driving display elements such as liquid crystals, and the like. Field effect transistors that are made to be thin-filmed in particular are called thin film transistors (TFTs). Further, a silicon TFT, that has an active layer formed from amorphous silicon that can be formed over a large surface area, is used in flat panel displays.
The development of TFTs, that use an In—Ga—Zn—O (hereinafter abbreviated as “IGZO”) oxide semiconductor thin film in the active layer (channel layer) instead of this amorphous silicon, has been carried out actively in recent years. An oxide semiconductor thin film can be formed at a low temperature and exhibits higher mobility that amorphous silicon, and further, because an oxide semiconductor thin film is transparent with respect to visible light, a flexible, transparent TFT can be formed on a substrate such as a plastic plate or a film or the like.
However, in order to fabricate a TFT that is IGZO and has high mobility, post-annealing at a high temperature (e.g., greater than or equal to 400° C. in the atmosphere) is needed, and it is difficult to form TFTs on flexible substrates that have low heat-resistance.
By the way, differently than an IGZO semiconductor thin film, an Sn—Ga—Zn—O (hereinafter abbreviated as “SGZO”) oxide semiconductor thin film, that uses Sn instead of the In in IGZO, does not contain In that is a rare metal, and Sn4+ has the same electron arrangement as In3+. Therefore, SGZO semiconductor thin films are anticipated as novel oxide semiconductor thin films that replace IGZO semiconductor thin films. Note that, among the aforementioned “SGZO”, the Ga does not have to be essential.
Thus, Japanese Patent Application Laid-Open No. 2010-50165 discloses a TFT in which a first metal oxide layer, that is formed on a gate insulating film and contains at least one element among In, Sn and Zn and the like, is made to be the active region, and a second metal oxide layer, that does not contain In and whose sheet resistance is higher than the first metal oxide layer, is layered on the first metal oxide layer.
Further, Japanese Patent Application Laid-Open No. 2010-16348 discloses a TFT having an interface stabilizing layer at at least one surface among the top surface and the bottom surface of an active layer that is structured by doping ZnO with ions of at least one among Ga, In, Sn or the like.
However, in the TFT disclosed in Japanese Patent Application Laid-Open No. 2010-50165, Sn is not essential as a structural element of the first metal oxide layer (active layer) that becomes the active region. Further, in this transistor, the first metal oxide layer is disposed between the gate insulating film and the second metal oxide layer in order to aim for stabilization of the threshold voltage, but it is thought that an improvement in mobility is not devised by such an arrangement.
Further, the TFT disclosed in Japanese Patent Application Laid-Open No. 2010-16348 has an interface stabilizing layer that can become an intermediate layer between the gate insulating film and the active layer, but Sn is not essential as a structural element of the active layer. Further, in Japanese Patent Application Laid-Open No. 2010-16348, the object is to aim for improvement of the interface characteristic, and improvement of the mobility of a TFT, that has an active layer structured by an SGZO oxide semiconductor, is not disclosed or suggested in any way.